This application is a continuation of Ser. No. 07/412,108 filed Sept. 25, 1989, which is a continuation-in-part of co-pending U.S. patent application Ser. No. 180,092, filed 11 Apr., 1988, now abandoned, which is a continuation-in-part of application Ser. No. 036,085, filed Apr. 9, 1987, now abandoned and is also a continuation-in-part of co-pending U.S. patent application Ser. No. 125,643, filed Nov. 25, 1987, now abandoned.
2. Field of the invention
The present invention pertains to forming complexly shaped articles by freeze-forming a non-aqueous slurry of inorganic solids, generally ceramic and/or metallic, by molding the slurry into a desired shape and then drying the frozen shape by non-destructive evaporation/sublimation to provide a green article with adequate green strength. The forming operation can be performed under low pressure and the green article can be conventionally sintered.
2. The state of the art
Complexly shaped, three-dimensional, high performance ceramic parts are essential structural and electronic components for a wide variety of industrial applications. High performance properties are, for example, strength, toughness, uniformity, surface finish, resistivity, optical properties, and thermal expansion. These and other properties are markedly affected by the quality of the starting material and the manner in which it is processed. Factors that have limited the production of advanced ceramics for high performance applications are (i) poor strength and reliability, stemming from poor raw material quality and improper processing techniques, and (ii) high cost, stemming from low product yields, long processing cycle times, and high capital equipment expenditures.
For example, a high strength, high performance alumina article is one which can be characterized by a fully densified body homogeneously composed of uniform submicron alumina grains. If a processing step introduces a texture or a defect of a critical size into the microstructure, typically about 20-50 .mu.m, a strength limiting flaw will have been created and will result in a severe departure from the intrinsic or high performance properties desired. Historically, ceramics have not been used for high performance applications due to poor or inhomogeneous starting materials and the inclusion of property limiting defects through inadequate processing, both as mentioned above. Only recently has the ceramics community recognized the importance of both the starting materials and the processing techniques on the properties of the article produced.
Typically, three-dimensional complexly shaped ceramic parts are manufactured by a process analogous to thermoplastic injection molding, in which a ceramic or metallic powder is compounded with a mixture of molten or solvated thermoplastic resins at high torque. The resulting mixture has a dough-like consistency, which is why the compounding process is generally referred to as "kneading." Homogeneous particle dispersion is difficult and tedious to obtain in such a system, and traditionally has been a source of microstructural defects, such as holes and non-uniform particle packing.
The resulting mixture is then molded using a high pressure injection molding machine. The molding machine and the molds used are typically large and expensive because injection pressures typically have an approximate range of 2500 psi to 30,000 psi, thus requiring mold clamping forces in the "tens of tons" range. The high viscosity and dough-like consistency of the molding composition can result in weld or knit lines and mold gate, sprue, and parting line textures, all of which can create property limiting defects.
After the part is molded, the thermoplastic/ceramic composition is subjected to binder removal, which is a long (typically requiring days), expensive, and deleterious process, particularly for a fine particle matrix typical of a high performance ceramic body. Initially, binder removal can result in bubble formation, delamination, and blistering of the part (as typically 40% by volume of the composite is a plastic material which is removed from a finely porous body). Binder removal is commonly practiced by heating the polymer/ceramic composite beyond the polymer softening point; accordingly, dimensional tolerance is difficult to control because of fluids escaping from the softening composite matrix; this liquefaction often coincides with the development of internal pressures due to gasification of the polymer (by depolymerization, vaporization, or pyrolysis reactions).
After binder removal, and for all processes in which a densified article is desired, the porous particulate body is sintered at high temperatures so that the particulates can fuse together; this reaction produces a dense, strong ceramic that is approximately 20% smaller than the presintered (green) particulate part. Final machining is generally required due to poor dimensional tolerances, parting lines, and gate remanents remaining on the fired part; unfortunately, the machining process commonly imparts defects to the fired part, thereby creating property limiting, especially strength limiting, defects.
An alternate approach to thermoplastic resin molding has been to substitute low temperature melting, low viscosity waxes in place of the thermoplastic resins also for low pressure injection molding; yet problems remain, such as those associated with dispersion, binder removal, machining, green strength, and dimensional tolerance.
Historically, investigators have recognized the limitations that the binder has placed on the processing of complexly shaped, three-dimensional parts. The art later began to understand and appreciate that the binder, which had allowed the ceramic and metal particles to be formed into a shape and later handled, was also the cause of many economic and performance problems. Rivers, as described in U.S. Pat. No. 4,113,480, developed an aqueous-based injection molding process exclusively for metal powders using 1.5 to 3.5 wt. % (metal powder basis) of high viscosity methylcellulose additive to provide green strength. The resulting mixture of metal powder, water, and methylcellulose has a "plastic mass" consistency and can be injection molded at 8,000 psi. The molded mass is then thermally dried and the green part is conventionally sintered. Although binder burnout was eliminated by this particular process, defects still remain, as well as the costs associated with dispersion and molding of a high viscosity mix and the implementation of a necessary but difficult thermal drying step. At present, there are a number of similar processes being used based on, for example, METHOCEL brand methylcellulose (available from Dow Chem. Co., Midland, Mich.). Another analogous process is taught by U.S. Pat. No. 4,734,237, in which Fanelli et al. describe using a slip having an agaroid gelling agent for molding ceramics; the molding pressures are between 20 psi and 3500 psi, and the as-molded parts can be dried or placed directly in the furnace. Comparably, Maeda et al., in Japanese laid open application 61-158403, describe molding a mineral spirits-based slip in which the temperature is lowered to below the melting point of the dispersant. Presumably, this is also a gellation process because of the low amount of dispersant (less than 5 wt. % in the examples, comparable to the amount of agar Fanelli et al. teach), and a melting point for the dispersant significantly greater than that for mineral spirits (also known as naphtha, benzin; m.p. -73.degree. C.).
The use of a molding vehicle which could be frozen has been investigated as an alternate method for casting or molding without the use of thermoplastic carriers. Sublimative drying by freeze drying (lyophilization) has been shown to be less destructive to the particle fabric in the green part during drying. A. Kwiatkowski et al., "Preparation of Corundum and Steatite Ceramics by the Freeze Drying Method," Ceramurqia International, vol. 6, no. 2, pp. 79-82 (1980). A closer examination of this disclosure reveals that what is ostensibly "freeze-drying" allows not only for some evaporation but also for the formation of a continuous liquid phase of liquid at the vehicle-atmosphere interface. An indication of the formation of a continuous liquid phase during drying is found in the description by Kwiatkowski et al. of smoother and more densely packed surfaces on their articles. Such characteristics indicate the formation of a continuous liquid phase during drying, which allows for capillary forces between the inorganic particles, thereby pulling them together during drying to yield a denser, smoother surface.
A method has been described by Nesbit, in U.S. Pat. No. 2,765,512, which involves casting a ceramic shape from a thick slip containing a hydrogen bonding medium (such as water), a cryoprotectant, and ceramic particles which are then frozen into a shape while in the mold. The resulting frozen part was demolded, dried at room temperature and pressure, and subsequently fired. Downing et al., U.S. Pat. No. 3,885,005, has cast coarse grained refractory shapes from a slip containing 70% coarser than #200 mesh ceramic particles, water, and a silica sol binder. The resulting cast shape was subsequently frozen, causing the silica to gel and cementing the refractory particles together. The residual water was frozen and the part was demolded and heated to 200.degree. F. to thaw and dry the part. Tomilov, G.M., and T.V. Smirnova, "Molding Quartz-Ceramic Articles Using an Aqueous-Slip Freezing Method," Glass & Ceramics, no. 10, pp. 655-6 (1977) also describe freeze molding a ceramic part that is later dried by the application of heat.
Dennery et al., U.S. Pat. No. 3,567,520, in making metal parts from powdered metals, formed an aqueous-based paste sheet into a part, which was then frozen at -60.degree. F., and subsequently freeze dried to overcome thermal drying stresses which would be destructive to the part. Maxwell et al., U.S. Pat. No. 2,893,102, cast and molded thicker parts from an aqueous ceramic slip in which the slip and mold were frozen in a CO.sub.2 bath followed by freeze drying and sintering.
As a slight departure from the art thus described, Weaver et al., U.S. Pat. No. 4,341,725, describes the use of a cryoprotectant as an additive in a hydrogen-bonding suspension medium to inhibit ice crystal growth to the order of 0.020-0.050 mm (i.e., 20-50 .mu.m); they teach that the defects induced by larger ice crystals can cause severe strength limiting defects. A paste-like slip at very high solids content (about 70 vol.%) is vibrated into a mold and frozen; the frozen part is dried in a vacuum oven. Weaver et al. claim that the foregoing prior art would result in "low performance" articles riddled with scars resulting from ice crystal formation.
What may be viewed as a combination of the gelation technology and the freezing technology is a disclosure by Blasch et al., in U.S. Pat. No. 4,552,800, in which freeze-sensitive colloidal sols are used as the solidification agent. Silica based sols are well-known to irreversibly gel upon such processes as heating and freezing. The gelled green articles, which may contain a substantial amount of silica, are typically vitreous sintered; for example, the composite alumina-zirconia articles made by Blasch et al. are underfired, they are fired below the conventional sintering temperature for either alumina or zirconia, but are heated so that the silica-based glass melts and causes densification of the body.
Takahashi, in European Patent Applications Nos. 160,855 and 161,494, describes a method for freeze-pressure molding inorganic powders. That method includes providing a flocculated feedstock, shaping the feedstock under high pressure (at least about 2800 psi (200 kg.sub.f /cm.sup.2)), consolidating the shaped feedstock (including removing a portion of the fluid medium), and freezing under pressure to form a frozen shape; the resulting shape is dried, such as by freeze drying, and then conventionally sintered. Besides the necessity for high pressure forming and consolidation, the Takahashi process has another disadvantage of being limited to particles having a size not greater than about 1 (one) micron. In general, a perusal of these two European patent applications shows that the process is designed to overcome or avoid volume expansion on freezing by the use of mechanical means. Still further, Takahashi achieves what is termed a "high density" article, which for alumina is disclosed to be about 85% of theoretical, far below the 98+% of theoretical density typical for high performance alumina applications. It is also noted that the Takahashi process is designed to consolidate the inorganic particles by forming a frozen shell at the mold surface; the consolidation apparently provides sufficient green strength that the part does not have to be frozen throughout, and may only be frozen near the mold surface.
One factor that effects whether a particular process possesses advantages or detriments over another process is the type and use of the article being made. Refractories, for example, are typically made from a very broad distribution of particle sizes (e.g., fines of colloidal size to a few hundred microns in size) and often include a not insignificant amount of silica. The resulting articles are usually not fully dense; the open, porous structure is desired because it provides increased resistance and immunity to thermal shocks.
Herrmann, in U.S. Pat. Nos. 3,330,892 and 3,234,308, (both incorporated herein by reference) describes a molding process using vehicles which are normally solid at room temperature, such as paradichlorobenzene, naphthalene, and camphor; the slip is heated to liquefy the vehicle and the mold is cooled to elicit solidification. The vehicles are dried by subliming at about 10.degree. C. below the melting point of the vehicle. This disclosure is notably directed to refractories, particularly allowing the pressing of dense refractory bricks from finely divided particles (-325 Tyler mesh, about 44 .mu.m). Assuming minimal volume change on drying (.ltoreq.1%) and based on the linear shrinkages during sintering, the fired densities described in the examples range from 73% to 87% of theoretical density, and 96% of theoretical density for a borosilicate glass powder composition. In some contrast, the fired density of ceramics for structural applications (e.g., non-glass compositions such as engine components) is typically at least 98% of theoretical.